The present invention relates generally to diaphragms for use in diaphragm piston assemblies, and in particular to diaphragms without a molded-in convolution.
A diaphragm convolution is the annular fold in a diaphragm that provides the diaphragm with sufficient length to accommodate piston travel through its full range of movement. The convolution may be molded-in or, in the case of conical-type diaphragms, the convolution is formed naturally when the diaphragm piston is assembled in place. The ideal diaphragm must have sufficient centerline length between its clamping points to allow a full loop convolution to form in a pressurized condition of the piston assembly. Otherwise, the forces on the diaphragm could cause the diaphragm to be pulled out of its clamping point or could cause such extreme diaphragm stress as to destroy the diaphragm.
Diaphragms having a molded-in convolution are not generally suited for the application of reinforcing fabric, due to the rather sharp radii the fabric must follow, particularly in the area of the diaphragm clamping bead. During vulcanization, the reinforcing fabric tends to revert to its natural flat shape, thus migrating toward the inside surface of the aforementioned radii. This results in the fabric so closely approaching the diaphragm surface as to render the diaphragm susceptible to ballooning under high pressure, with consequent early failure. Without resorting to the expense of custom designed fabric to avoid this problem, the recognized advantage of long diaphragm service life, therefore, is unrealized in diaphragms having a molded-in convolution.
On the other hand, conical-type diaphragms are generally better suited for use with reinforcing fabric, since the diaphragm skirt portion is molded without any convolution. However, conical-type diaphragms are subject to distortion and high stresses, since these diaphragms are unconvoluted in their natural state, but assume a convoluted configuration throughout the operating range of travel of a piston with which the diaphragm is assembled. These distortions and stresses may be defined as the change in diameter that a given point of the diaphragm undergoes during transition between extreme operating positions, and in addition to causing premature diaphragm failure, also tend to produce what is commonly known as "spring effect", in which condition the diaphragm tends to revert to its molded shape.